The standard E coupler has been in use on the railroads for over 40 years and has performed very satisfactorily during this period. An essential requirement of a railway car coupler is that upon completion of a coupling operation, the knuckles of the coupled couplers must be locked in closed position so as to avoid a train parting. In the standard E coupler, the coupler head is provided with stop means intended to guide the lock as it drops by gravity to its locking position during a knuckle closing operation. However, due to the very irregular configuration of the lock, the lock chamber in the coupler head and the knuckle, the lock under certain coupling conditions may fail to drop to its full locking position. Instead of dropping, the lock may be hung-up on the stop means and be prevented from moving to its normal locking relation with the closed knuckle. In its hung-up position, the rear lower end of the lock body may rest on the stop means disposed within the lock chamber on the guard arm side of the coupler and thereby retain the lock out of locking relationship with the knuckle, and, although the knuckle is closed, it remains unlocked. The present invention is concerned with providing a lock configuration whereby the lock, when applied to the present standard E coupler, will avoid the aforementioned hang-up of the lock and assure dropping thereof to full locking position upon closing of the knuckle, as during a coupling operation.